1. Field of the Invention
The self-tensioning printing saddle lock of the present invention relates to a locking device for securing a photosensitive printing plate to a printing cylinder in a rotary printing press.
2. Description of the Prior Art
In the printing industry it has been common practice to provide flexible metallic photosensitive printing plates having a raised image on one side thereof for reproduction as such plate travels around with a printing cylinder in a rotary printing press. Since such metallic plates are not reusable, the expense thereof to a company printing numerous different flats or pages of images is considerable. This is particularly true for the newspaper industry where 50 or even 100 different pages of print and artwork reproduction may take place daily. Efforts have been made to devise paper printing plates which can be manufactured for less than half the cost of metallic photosensitive plates. However, such paper printing plates are typically dimensionably unstable when exposed to different humidity and temperature levels. Consequently, the circumferential dimension of such paper plates may vary as much as 1/16 of an inch in the circumferential direction as it travels about with such cylinder. Such variations in printing plate dimensions render present day locking devices incorporated in rotary printing cylinders unsatisfactory for holding such paper printing plates firmly on the cylinders since such locks cannot accommodate such variations in length. Printing saddle locks have been proposed which include a floating tensioning bar biased to a central or neutral position by means of a single spring and then incorporate circumferentially extending alignment grooves for receipt of alignment screws to roughly maintain the tensioning bar parallel to the saddle edge. A device of this type is shown in U.S. Pat. No. 4,154,167, assigned to the assignee of the instant application. Such saddle locks, while being satisfactory for dimensionally stable printing plates, have proven unacceptable for dimensionally unstable printing plates since it is economically unfeasible to form the alignment pins and slots with the necessary precision to maintain the required tension bar alignment to provide uniform tensioning force to the opposite sides of a printing plate which may progressively change in dimension during the printing operation. Consequently, there exists a need for a self-tensioning printing saddle lock which will maintain a certain degree of tension on the paper printing plate itself as the dimensions thereof vary to thus maintain the printing plate under tension to closely fit the periphery of the printing saddle irrespective of such variations in dimensions. Although various printing cylinder lockup systems have been in use for more than 15 years on many different types of printing presses for locking printing plates thereon, the need for a self-tensioning on-cylinder lockup system has not been satisfied. The self-tensioning lock of the present invention not only accommodates the dimensional instability of paper plates, but the plates may be mounted thereon with either end at the lead end of the saddle.